When writing and naming ionic compounds when there are polyatomic ions present, you follow the same guidelines as if it were just two element ions.
Iron III Nitrate - write the two ions named first, then switch the charges. Use parentheses to trap the polyatomic ion inside. The subscript outside of the parentheses tells us that you need 3 nitrate ions to bond with 1 iron III ion.
Ammonium chloride - ammonium is the polyatomic ion here and you'll want to really ingrain it into your head so you recognize it. Since both charges are "1" you don't need to add any subscripts. You can, however, write (NH4)Cl if it helps you in your learning.
Recognize that the parenthesis are there to keep the polyatomic ion grouped together. Don't try to separate elements out of the parentheses.
At times you will have to break apart the ions in a compound to double check which transition element you have:
CuO --- Copper II Oxide -- We know that oxygen has a -2 charge. Copper must have a +2 charge because there are no subscripts in the formula
Cu2O -- Copper I Oxide --- since there was no reduction done from the oxygen, the copper charge didn't match
- At times you will have to break apart the ions in a compound to double check which transition element you have:
CuO --- Copper II Oxide -- We know that oxygen has a -2 charge. Copper must have a +2 charge because there are no subscripts in the formulaCu2O -- Copper I Oxide --- since there was no reduction done from the oxygen, the copper charge didn't match